Not Applicable
The present invention is directed generally to optical transmission systems. More particularly, the invention is directed toward optical transmission systems including higher performance optical amplifiers.
Digital technology has provided electronic access to vast amounts of information. The increased access has driven demand for faster and higher capacity electronic information processing equipment (computers) and transmission networks and systems to link the processing equipment.
In response to this demand, communications service providers have turned to optical communication systems, which have the capability to provide substantially larger information transmission capacities than traditional electrical communication systems. Information can be transported through optical systems in audio, video, data, or other signal format analogous to electrical systems. Likewise, optical systems can be used in telephone, cable television, LAN, WAN, and MAN systems, as well as other communication systems.
Early optical transmission systems, known as space division multiplex (SDM) systems, transmitted one information signal using a single wavelength in separate waveguides, i.e. fiber optic strand. The transmission capacity of optical systems was increased by time division multiplexing (TDM) multiple low bit rate, information signals into a higher bit rate signals that can be transported on a single optical wavelength. The low bit rate information carried by the TDM optical signal can then be separated from the higher bit rate signal following transmission through the optical system.
The continued growth in traditional communications systems and the emergence of the Internet as a means for accessing data has further accelerated the demand for higher capacity communications networks. Telecommunications service providers, in particular, have looked to wavelength division multiplexing (WDM) to further increase the capacity of their existing systems.
In WDM transmission systems, pluralities of distinct TDM or SDM information signals are carried using electromagnetic waves having different wavelengths in the optical spectrum, typically in the infrared portion of the spectrum. The pluralities of information carrying wavelengths are combined into a multiple wavelength WDM optical signal that is transmitted in a single waveguide. In this manner, WDM systems can increase the transmission capacity of existing SDM/TDM systems by a factor equal to the number of wavelengths used in the WDM system.
Optical WDM systems were not initially deployed, in part, because of the high cost of electrical signal regeneration equipment required approximately every 20-50 km to compensate for signal attenuation for each optical wavelength throughout the system. The development of the erbium doped fiber optical amplifier (EDFA) provided a cost effective means to optically amplify attenuated optical signal wavelengths in the 1550 nm range. In addition, the 1550 nm signal wavelength range coincides with a low loss transmission window in silica based optical fibers, which allowed EDFAs to be spaced further apart than conventional electrical regenerators.
The use of EDFAs essentially eliminated the need for, and the associated costs of, electrical signal regeneration/amplification equipment to compensate for signal attenuation in many systems. The dramatic reduction in the number of electrical regenerators in the systems, made the installation of WDM systems in the remaining electrical regenerators a cost effective means to increase optical network capacity.
WDM systems have quickly expanded to fill the limited amplifier bandwidth of EDFAs. New erbium-based fiber amplifiers (L-band) have been developed to expand the bandwidth of erbium-based optical amplifiers. Also, new transmission fiber designs are being developed to provide for lower loss transmission in the 1380-1530 nm and 1600-1700 nm ranges to provide additional capacity for future systems.
Raman fiber amplifiers (xe2x80x9cRFAsxe2x80x9d) are also being investigated for use in wide bandwidth, e.g., 100 nm, optical amplifiers, but RFAs generally make less efficient use of pump power than EDFAs. Therefore, RFAs have not been deployed in commercial systems because significant pump powers on the order of hundreds of milliwatts are required to achieve the required levels of amplification.
RFAs do, however, have appeal as a viable option for next generation optical amplifiers, because RFAs provide low noise, wide bandwidths, and wavelength flexible gain.
Commonly assigned U.S. patent application Nos. 09/119,556 and 09/253,819, which are incorporated herein by reference, describe RFAs that can be deployed in existing fiber optic networks having various fiber designs and compositions and over a wide range of signal wavelengths.
RFAs are theoretically scalable to provide amplification over a range of bandwidths and power. However, the amplification bandwidth and power is limited, in part, by the amount of pump power that can be delivered to the fiber amplifier. The capability to provide higher pump powers is essential for continued development of optical amplifiers and optical systems to meet the requirements of next generation optical systems.
The systems, apparatuses, and methods of the present invention address the above needs to provide higher performance optical amplifiers and systems. The optical systems generally include at least one optical transmitter configured to transmit information via at least one optical signal wavelength, or channel, to at least one optical receiver via optical transmission media, such as an optical fiber. The system will also include at least one optical amplifier disposed between the transmitters and receivers to overcome various signal power losses, such as media attenuation, combining, splitting, etc. in the system.
The optical amplifier will generally include an optical signal amplifying medium supplied with pump power in the form of optical energy in one or more pump wavelengths via an optical pump source. The pump source can include one or more optical sources, such as narrow and broad band lasers or other coherent, as well as incoherent sources.
The optical amplifier will further include a pump amplifier configured to amplify the pump power being supplied to the signal amplifying media. In various embodiments, the pump amplifier includes a pump amplifying medium supplied with pump booster power in the form of optical energy from a pump booster source. The pump amplifying medium can include various amplifying fibers as may be appropriate for amplifying the pump power. The pump amplifier can be configured to provide Raman amplification of the pump power being supplied to at least one amplifying media to optically amplify signal wavelengths passing through the amplifying media. For example, the pump booster power can be supplied in the 1300-1450 range to provide Raman amplification of pump wavelengths in the 1400-1500 range in the pump amplifier.
In addition, the pump booster power can be split and used to amplify pump power being supplied to multiple optical amplifiers disposed along one or more transmission fibers. In this manner, the pump booster power, which can be several watts, and the cost of the pump booster source can be spread over a number of amplifiers in the system. It may be also be desirable to combine the power from two or more pump booster source prior to splitting the pump booster power to amplify the RFA pump wavelengths to provide additional redundancy in the system.
In various embodiments, a cascaded Raman resonator (xe2x80x9cCRRxe2x80x9d) and/or semiconductor laser diodes can be used as the pump booster source to provide pump booster power to amplify the pump power provided by the pump sources. The pump power supplied by each of the optical sources in the pump source can be varied to control the overall pump power distribution over the pump wavelength range.
In various CRR embodiments, CRR input power in at least one input wavelength is introduced into a fiber Raman resonator ring via an input wavelength division multiplexing (WDM) coupler. An output WDM coupler is further coupled to resonator ring to output the pump booster power at an appropriate wavelength to supply optical energy for use in the pump amplifying fiber.
The use of pump amplifier external to the transmission fiber in the present invention provides flexibility in the optical amplifier design. For example, lower power optical sources can be employed in the pump sources, thereby reducing component costs in the system. The optical sources can be individually controlled and fine tuned before the pump power is amplified. When Raman pump amplifiers are used, the wavelength and relative power of the optical sources can be varied within the Raman gain bandwidth of the pump amplifier to vary the wavelength profile of the pump power provided to the amplifier without changing the pump amplifier. Thus, the optical amplifiers of the present invention provide increased power, control, flexibility, and upgradability necessary for higher performance optical systems. These advantages and others will become apparent from the following detailed description.